Specialists from 24 Commando Engineer Regiment in Devon were on the ground in Somerset on Thursday morning, patrolling in Land Rovers.

One officer said they were on reconnaisance, to see what help they could offer.

He said: "We are just looking at the kind of areas under threat and specifically routes in and out of the area and looking at what we might be able to do to assist the local authority."

Defence sources said up to 100 servicemen, largely Royal Marines and Royal Engineers, were on standby at nearby Norton Manor Camp in Somerset and were expected to move into the flooded area later on Thursday. The troops are equipped with Land Rovers and lorries.

The deployment of military personnel to Somerset came as forecasters warned further heavy rains could fall in the area on Thursday.

Many parts of the Levels have been flooded since Christmas and there are fears it could be many months before the water is completely pumped away.

Chris Burton, a forecaster for MeteoGroup, said up to one inch could fall across the Somerset Levels on Thursday, with strong winds of up to 60mph.

Mr Paterson was met with hostility when he visited Somerset on Monday, with farmers, politicians and church leaders demanding immediate action to alleviate what some residents described as "Third World" conditions.

On Wednesday he said the county council had only asked for assistance "for the first time today".

Members of 24 Commando Engineer Regiment (54 Commando Squadron) drive through flood water in Burrowbridge, Somerset

But a council spokesman said the authority had been discussing with the military for weeks the possibility of bringing in manpower, sandbags and amphibious vehicles.

John Osman, the Conservative leader of Somerset County Council, said military help would give beleaguered residents the chance to repair their battered properties.

The village of Muchelney has been effectively isolated since the turn of the year due to flooded roads.

Mr Paterson will chair another meeting of the Cobra committee on Thursday afternoon to discuss the flooding situation, Downing Street disclosed.

David Cameron's official spokesman was asked whether the Prime Minister felt that the Environment Agency's approach to the crisis had been inadequate, amid criticism by residents and MPs of its response.

Mr Cameron's warning on Wednesday that preparations for dredging rivers must be speeded up had appeared to act as a rebuke to the quango, which had earlier played down the importance of using the technique to alleviate the flooding.

The spokesman said: "A number of agencies, including the Environment Agency, have been working hard in response to the situation which communities are facing.

"On dredging, as the Prime Minister said, that needs to be done as soon as it is safe to do so, and that will be done. And then I'm sure, as you would expect, once we have helped the communities through the immediate challenges that they face, we will across government, working with local authorities, look at what lessons can be learnt."

The spokesman denied that there was a dispute with the Environment Agency over the need for dredging once the floods abate.

"The decision around dredging immediately, once the water levels make it safe to do so, is one that we think certainly is very important and it's one that the Environment Agency agree with."

Asked whether the Prime Minister had confidence in Environment Agency chairman Lord Smith, the spokesman said: "I think it is right that he continues to provide leadership to the Environment Agency.

"The Environment Agency has a very important role alongside other agencies in responding to the desperate situation we see in Somerset, as well as the wider repairs and ongoing investment in other parts of the country that have suffered over the recent period."